A device under test (DUT) power supply (DPS) is typically employed to provide a constant output voltage for testing integrated circuits. The DPS should provide an accurate output voltage while sourcing and/or sinking a wide range of currents to the DUT. In addition, the DPS should respond quickly to transient currents without compromising the accuracy of the output voltage. Further, the DPS should be able to drive a wide range of capacitive loads and remain stable.
To achieve the desired accuracy, DPS circuits are typically closed loop systems. Because of the limitations of closed loop response times, a DPS typically employs large external capacitances to help provide transient currents to the DUT. Providing a stable solution under large capacitive loads can be very difficult. Under large capacitive loads, the DPS provides large amounts of current to recharge the external capacitance. In addition, the large external capacitance is commonly switched out of the circuit to enhance low current measurements when the capacitance is not needed. Therefore, the DPS typically needs to be stable under light capacitive load conditions as well as under large capacitive load conditions.
In addition to providing the accurate output voltage, the DPS should also accurately measure the currents it sources and/or sinks to the DUT without affecting the accuracy of the output voltage. To measure the current provided to the DUT accurately, several ranges are typically used. The ranges should be able to be changed to search for the measured current without affecting the accuracy of the output voltage. When the accuracy of the output voltage is affected, it is termed glitching.
Typical DPS circuits include an amplifier and a series sense resistor or resistors. A bypass capacitor is coupled across the sense resistor or resistors and a load capacitor is coupled between the DUT and common or ground. An input forcing voltage and the forced output voltage supplied to the DUT are provided to the amplifier. In response to the forcing voltage and forced output voltage, the amplifier adjusts the forced output voltage to equal the forcing voltage. The current supplied to the DUT is measured by measuring the voltage drop across a sense resistor. The load capacitor is selected to keep the DUT forced output voltage constant by providing additional transient current to the DUT when the bandwidth of the DPS is insufficient. The load capacitor introduces a pole. The bypass capacitor is used to lower the sense resistor impedance at higher frequencies by introducing a zero. Unless the bypass capacitor is on the order of the load capacitor, the zero does not completely cancel the pole. Also, as the bypass capacitor gets larger, another pole is introduced.
During range switching, glitching is typically a major issue. Since the output of the amplifier is slow to react, an instantaneous current is provided during range switching. This current causes a voltage excursion until the loop can respond. In addition, in a selected range, the current is limited to a maximum value. Therefore, the typical DPS has several disadvantages. First, changing the measure current range can cause aberrations or glitches in the forced output voltage due to the opening and closing of the loop. Second, in low current ranges, a large series resistance is used. Since the load capacitor is usually large, a low frequency pole is introduced that when added to the pole of the amplifier can cause the system to become unstable. The bypass capacitor should be carefully selected to avoid instability by introducing a zero. Third, the low range current is limited by the range resistor. In all but the highest current range, the circuit cannot provide maximum current. Finally, current limiting is range dependent.
For these and other reasons, there is a need for the present invention.